1. Field of the Invention
The present invention relates to a red phosphorescent compound and an organic electroluminescent device (OELD) and more particularly to a red phosphorescent compound having high color purity, high luminescent efficiency and improved lifetime and an OELD using the red phosphorescent compound.
2. Discussion of the Related Art
Recently, a requirement for a flat panel display device having a relatively large display area and a relatively small occupancy has been increased. Among the flat panel display devices, an OELD has various advantages as compared to an inorganic electroluminescent device, a liquid crystal display device, a plasma display panel, and so on. The OELD device has excellent characteristics of viewing angle, contrast ratio, and so on. Also, since the OELD device does not require a backlight assembly, the OELD device has low weight and low power consumption. Moreover, the OELD device has advantages of high response rate, low production, cost and so on.
In general, the OELD emits light by injecting electrons from a cathode and holes from an anode into an emission compound layer, combining the electrons with the holes, generating an exciton, and transforming the exciton from an excited state to a ground state. A flexible substrate, for example, a plastic substrate, can be used as a base substrate where elements are formed. The OELD has excellent characteristics of viewing angle, contrast ratio, and so on. Also, since the OELD does not require a backlight assembly, the OELD has low weight and low power consumption. Moreover, the OELD has advantages of a high response rate, low production cost, high color purity, and so on. The OELD can be operated at a voltage (e.g., 10V or below) lower than a voltage required to operate other display devices. In addition, the OELD is adequate to produce full-color images.
A general method for fabricating OELDs will be briefly explained below. First, an anode is formed on a substrate by depositing a transparent conductive compound, for example, indium-tin-oxide (ITO). Next, a hole injection layer (HIL) is formed on the anode. For example, the HIL may be formed of copper phthalocyanine (CuPC) and have a thickness of about 10 nm to about 30 nm. Next, a hole transporting layer (HTL) is formed on the HIL. For example, the HTL may be formed of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (NPB) and have a thickness of about 30 nm to about 60 nm. Next, an emitting compound layer (EML) is formed on the HTL. A dopant may be doped onto the EML. In a phosphorescent type, the EML may be formed of 4,4′-N,N′-dicarbazole-biphenyl (CBP) and have a thickness of about 30 nm to about 60 nm, and the dopant may includes one of iridium complex represented by following Formulas 1-1 to 1-3.

Next, an electron transporting layer (ETL) and an electron injection layer (EIL) are stacked on the EML. For example, the ETL may be formed of tris(8-hydroxy-quinolate)aluminum (Alq3). A cathode is formed on the EIL, and a passivation layer is formed on the cathode.
In the above structure, the EML produces red, green and blue colors such that the OELD can display full-color images. In an emitting compound, an exciton is generated by combining the electrons from a cathode and holes from an anode. The exciton includes a singlet exciton and a triplet exciton. The singlet exciton participates in a fluorescent type emission, while the triplet exciton participates in a phosphorescent type emission. The singlet exciton has a formation probability of about 25%, while the triplet exciton has a formation probability of about 75%. Accordingly, the phosphorescent type emission has luminescence efficiency greater than the fluorescent type emission.
In the phosphorescent compound, since a red phosphorescent compound has excellent luminescence efficiency as compared to a red fluorescent compound, the red phosphorescent compound has been widely developed and researched to improve an emission efficiency of the OELD. The phosphorescent compound is required to have high luminescence efficiency, high color purity, long life span, and so on. Particularly, as shown in FIG. 1, as the color purity of an OELD using a red phosphorescent material becomes higher (i.e. as the X index on CIE chromaticity coordinates increase), the relative spectral sensitivity of images from the OELD decreases. Accordingly, it is difficult to achieve high luminance efficiency of the OELD.